1. Field of the Invention
The present invention relates to a urea synthesis method comprising separating unreacted ammonia and unreacted carbon dioxide from the reaction mixture by stripping the mixture with the entire carbon dioxide feedstock, at a pressure substantially equal to the urea synthesis pressure.
2. Description of the Related Art
Methods for preparing urea in which ammonia is reacted with carbon dioxide, are well known, and a variety of proposals have been made for urea synthesis methods. For example, ammonia may be reacted with carbon dioxide in a urea synthesis zone at a desired urea synthesis pressure and urea synthesis temperature, and the unreacted ammonium carbamate may then be separated from the product urea synthesis solution as a mixed gas of ammonia and carbon dioxide. This mixed gas may then be absorbed in an absorbing medium and recirculated to the urea synthesis zone, thereby providing a urea solution from which the ammonium carbamate has been separated from the urea synthesis solution.
Generally, a urea plant consists of a synthesis column, a carbamate condenser and a stripper. Often, the bottoms of the synthesis column and the carbamate condenser are placed at a height of 20 to 30 meters above the ground. In this case, the supporting structure is composed of concrete was laid up to this height, or a combination of concrete with a steel framed structure was built, rather than solely a steel-frame structure. In light of the time and effort required for the installation and maintenance of such apparati, urea synthesis processes with urea synthesis columns and the like placed on the ground have been developed.
The present inventors have previously proposed (Japanese Patent Laid-Open No. 10-182587) an improved method of synthesizing urea, comprising stripping unreacted ammonia and carbon dioxide from the product mixture with the carbon dioxide feedstock. This method allows the equipment to be placed on the ground, rather than elevated as described above. This method comprises providing a vertical condenser above the urea synthesis column so that the mixed gas from the stripper contacts the absorbing medium under cooled conditions, in order to condense the mixed gas. The apparatus in which this method is practiced has a first down-flow-pipe passing from the top of the condenser to the bottom of the synthesis column in order to allow the resulting condensate to flow down to the bottom of the synthesis column by means of gravity. This condensate liquid, along with a part of the liquid ammonia or carbon dioxide starting materials participate in the synthesis of urea. The urea synthesis solution, provided by means of a second down-flow-pipe having an opening in the top of the synthesis column, is introduced into the stripper by means of gravity. The unreacted ammonia and carbon dioxide, i.e., the above-described mixed gas, is separated with the remainder of the carbon dioxide starting material and introduced into the bottom of the above-described condenser and a condensed, or the condensate liquid is sucked from the vertical condenser into the bottom of the urea synthesis by means of an ejector using preheated liquid ammonia starting material as a driving fluid to promote the synthesis of urea from the condensate.
However, in the above-described method, the ammonia and carbon dioxide separated at the top of the condenser are absorbed by the absorbing medium in the scrubber, without being recycled to the bottom of the urea synthesis column, and thus do not add heat to the urea synthesis column (i.e., because the heat of formation of ammonium carbamate due to the reaction between ammonia and carbon dioxide is not generated in the urea synthesis column). Thus, it is desirable to produce ammonium carbamate in the urea synthesis column by introducing a portion of the carbon dioxide feedstock into the urea synthesis column, in order to raise its temperature by means of the heat of the reaction.
Stamicarbon B. V. has also proposed a method in which the synthesis of urea and absorption of uncondensed gas is performed in one horizontal apparatus using a horizontal condenser combined with a horizontal urea synthesis reactor. The mixed gas containing ammonia and carbon dioxide from the stripper may be condensed to produce ammonium carbamate, in order to recover the heat formed at this time.
However, in this method, since the liquid flows only in the horizontal direction and the gas flows only upward, the liquid-gas contact is insufficient and the condensation rate of the gas remains low. Also, an almost ideal plug flow cannot be obtained, and therefore the rate of urea synthesis is low, thereby increasing the required capacity of this horizontal apparatus per unit production. Also, a large surface area is required for installing a horizontal apparatus. Furthermore, in order for the gas to flow uniformly in a horizontally oriented apparatus, a gas dispersion unit with a large pressure loss is required. There are also various additional problems, such as the large space required for liquid-gas separation.